The relative risk of the presently used general anesthetics is not known. Especially lacking is information about the effects of anesthetics on energy utilization by the heart. This study evaluates, by thermodynamic means, the actions of several general anesthetics on the heart in terms of their direct effects on energy utilization associated with tension generation, activation (Ca ions transport) and rest, and upon the overall enthalpic efficiency of contraction of the heart. Since these factors figure importantly in determining the total energy requirements of the heart, this information will be very helpful to the safe conduct of anesthesia in patients with ischemic heart disease where myocardial energy reserves are limited. BIBLIOGRAPHIC REFERENCES: Site of Action of Halothane to Depress Myocardial Contractile Force. B.F. Rusy. Accepted for presentation at 6th World Congress of Anesthesiology. Mexico City, April, 1976. Effect of General Anesthetics on the Activation Process in Cardiac Muscle: A Study of the Mechanism of Anesthetic-Induced Myocardial Depression. B.F. Rusy. In: Molecular Mechanisms of Anesthesia". Ed. B.R. Fink. Progress in Anesthesiology, Vol. 1, Raven Press, New York, 1975.